Conventional cellular area systems are typically deployed with mobile terminals that have the capability to mitigate interference. Mitigating interference has the effect of improving the signal to interference plus noise ratio (SINR) measured at the output of the receiver, resulting in better performance. This capability can be used, for example, in detecting transmitted data information or in computing some measure of channel quality information (CQI) that is used for link adaptation and user scheduling.
Interference mitigation can be accomplished by different methods. One approach is to cancel the interference from the received signal. Another is to jointly detect both a desired and an interfering signal, which improves the detection of the desired signal. In both cases, some parameters must be estimated with respect to the interfering signal.
Interference cancellation techniques commonly compute an impairment covariance that is used to cancel interference in interference rejection combining (IRC), generalized Rake (GRAKE), or successive interference cancellation (SIC) receivers. Estimation of an impairment covariance comprises, for example, removing the contribution of the desired signal from the received signal and forming the impairment covariance. Examples of this approach include known reference pilots available for channel estimation, or a detected data stream. When the interference is stationary over many samples in time and/or in frequency, averaging (e.g. filtering, smoothing, etc.) can be used to improve the impairment covariance estimation by lessening the effects of random noise.
If joint detection is used to mitigate interference, parameters used for detecting the desired and interferer signals must be obtained. For example, with coherent demodulation, an estimate of the interferer's channel response might be required along with the desired signal's channel response. These could be obtained if the pilot signals are known for both the desired and interfering signal. Alternatively, if only the desired signal's pilots are known, the interferer channel could be estimated using blind techniques. See U.S. Pat. No. 6,832,080, for example.
Co-channel interference itself can arise from different sources. For example, in systems using multi-stream transmission, such as multiple-input, multiple-output (MIMO) approaches, the different streams interfere with each other. However, both streams are considered part of the desired signal transmission to be sorted out at the receiver. Additionally, there is co-channel, inter-cell interference from other cells, which is undesired.
As today's systems transition to packet networks, data transmission may be discontinuous in time. A user's transmission time and duration depends on the amount of data required to be sent as well as the scheduling of the users by the system. The implication of this is the interference environment presented to the base or mobile is also discontinuous. Further, with Orthogonal Frequency Division Multiplex (OFDM) systems that allow the scheduling of users to take place in both the time and frequency domains, interferer discontinuities may exist in either domain.
The issue of discontinuous interference affects both link adaptation and data detection, and the effect is two-fold. First, when the interference is temporally discontinuous, the estimate of the impairment covariance may not be accurately obtained (e.g. the averaging time required to compute an accurate impairment estimate may be longer than the time the interference is stationary). Temporal discontinuities degrade both the detection quality and the estimation of the SINR used in link adaptation. Second, even in the case where the interferer parameters can be accurately estimated at a certain time, the interference can change between when they are estimated and when the data for the scheduled user is transmitted. Changes in the type or nature of interference results in a mismatch in the selected modulation and coding schemes (MCSs) and leads to detection errors or lower user throughputs.
In an OFDM system, such as LTE, the interference (and thus the SINR) is expected to be localized over the time/frequency dimensions due to the temporal fading and frequency dispersion as well as from the scheduling of users. For example, two users may be scheduled in one LTE sub-frame, such that one user's signal dominates the interference for a portion of the sub-frame, while the other user's signal dominates the interference for another portion of the sub-frame. In addition, there may be overlapping areas and areas where neither user presents interference. Thus, computing parameters for each interferer region should only use the received signal corresponding to that region.
Another example of interferer discontinuities is when an interferer signal is transmitted with different rank assignments, for example, due to the use of different pre-coding schemes. The rank of the interferer signal might be different in different parts of the signal bandwidth, causing interferer discontinuities in frequency.
In some parts of the time-frequency grid, a receiver that does not cancel interference may be preferred. However, in other parts of the grid, a receiver that cancels interference may be preferred. In theory, such operation can be accomplished, for example, by having one receiver that operates under different parameter settings—one for a noise only impairment and another for an impairment that includes interference. In practice, however, the difficulty lies in choosing when and where to use which receiver.
An alternative is an approach that selects between single-user demodulation or two-user (joint) demodulation and is described for a narrow-band TDMA system in U.S. Pat. Nos. 7,016,436 and 6,832,080. In these approaches, a determination is made using the transmitted pilot symbols as to which of the two receivers should be used for data detection. Under certain conditions (e.g. when the desired signal to noise ratio is low or there is no interference) single user demodulation is selected. When conditions are favorable, joint detection is used as a means for canceling the interferer transmission. However, the decision about which demodulation technique to use is made using the desired signal's training sequence and prior to data demodulation.
Another alternative is described in U.S. Pat. No. 7,440,489 B2 for a spread-spectrum system (e.g. WCDMA). In this approach, a first demodulation is performed on a de-spread signal. A second demodulation is performed either in parallel or in succession and one of the demodulated signals is selected depending on quality measures for the first and second demodulation. One of the detected signals is selected for further decoding.
Such approaches perform demodulation on all of the signal's samples using one of the available demodulators. This can be performed since there is a training or pilot sequence in time that allows for the impairment parameters to be estimated prior to demodulation. For an OFDM system such as LTE, a two-dimensional grid of symbols is transmitted with pilots spaced sparsely throughput this grid. Since the interference is localized within this grid, reliable estimation of the interference parameters becomes more difficult. For example, pilot symbols are spaced six sub-carriers apart in frequency and three, four or seven OFDM symbols apart. Thus, detecting interferer discontinuities using only reference pilots alone is made more difficult by the sparseness of the pilots.